In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over a radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data to the user equipments in downlink (DL) transmissions.
The possibility of identifying a geographical location of a user equipment in the radio communications network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the positioning application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g. 300 meters in Federal Communications Commission (FCC) Enhanced 9-1-1 in United States.
Positioning Overview
Several positioning methods for determining the location of a target device, which can be any of the wireless device or UE, mobile relay, Personal Digital Assistant (PDA), smartphone, wireless device for machine type communication, aka machine to machine communication, laptop mounting wireless devices or equipment, etc exist. The position of the target device is determined by using one or more positioning measurements, which can be performed by a suitable measuring node or the target device. Depending upon the positioning method used the measuring node can either be the target device itself, a separate radio node, i.e. a standalone node, serving and/or neighboring nodes of the target device etc. Also depending upon the positioning method the measurements can be performed by one or more types of measuring nodes.
The LTE architecture explicitly supports location services, e.g. see FIG. 1, by defining the Evolved Serving Mobile Location Center (E-SMLC) that is connected to the core network, i.e. Mobility Management Entity (MME) via the so called Location Service-Application Protocol (LCS-AP) interface and the Gateway Mobile Location Center (GMLC) that is connected to the MME via the standardized Lg interface. The LTE system supports a range of methods to locate the position of the target devices, e.g. UEs, within the coverage area of the Radio Access Network (RAN). These methods differ in accuracy and availability. Typically, satellite based methods, such as Assisted Global Navigation Satellite System (GNSS), are accurate with a (few) meter(s) of resolution, but may not be available in indoor environments. On the other hand, Cell Identity (ID) based methods are much less accurate, but have high availability. Therefore, LTE uses Assisted-Global Positioning System (A-GPS) as the primary method for positioning, while Cell-ID and Observed Time Difference of Arrival (OTDOA) based schemes serve as fall-back methods.
In LTE the positioning node, aka Evolved Serving Mobile Location Center (E-SMLC) or location server, configures the target device, e.g. UE, eNode B or a radio node dedicated for positioning measurements, e.g. location measurement unit (LMU), to perform one or more positioning measurements depending upon the positioning method. The positioning measurements are used by the target device or by a measuring node or by the positioning node to determine the location of the target device. In LTE the positioning node communicates with UE using LTE positioning protocol (LPP) and with eNode B using LTE positioning protocol annex (LPPa).
The well-known positioning methods used in cellular systems, e.g. LTE, are described below:
                Satellite based methods: In this case the positioning measurements are performed by the target device on signals received from the navigational satellites are used for determining the target device's location. For example either GNSS or A-GNSS, e.g. A-GPS, Galileo, COMPASS, Galileo and Additional Navigation Satellite System (GANSS) etc, measurements are used for determining the UE position. In A-GNSS the location or positioning server provides assistance data to the target device enabling it to perform measurements on the received GNSS signals with greater precision and faster. In target device based A-GNSS method, UE based A-GNSS, the target device performs the A-GNSS measurements by using the received assistance information and also determines its location itself, On the other hand in case of target device assisted A-GNSS method, aka UE assisted A-GNSS, the target device performs the A-GNSS measurements by using the received assistance information and signals the A-GNSS measurement results to the positioning server, which is E-SMLC in LTE. The positioning server uses these received A-GNSS measurements to determine the location of the target device.        OTDOA: This method uses UE measurements related to time difference of arrival of signals from radio nodes, e.g. Reference Signal received Time Difference (RSTD) measurement in LTE, for determining UE position in LTE or SFN-SFN type 2 in High Speed Packet Access (HSPA). To speed up OTDOA measurements and also to improve their accuracy, the positioning server provides OTDOA assistance information to the target device. The OTDOA can also be UE based or UE assisted positioning method. In the former the target device determines its location itself whereas in the latter the positioning server, e.g. E-SMLC, uses the received OTDOA measurements from the target device to determine the location of the target device.        Uplink-Time Difference of Arrival (UTDOA): It uses measurements done at a measuring node, e.g. LMU, on signals transmitted by the target device. The LMU measurements from multiple LMUs are used by the location server, e.g. E-SMLC in LTE, for determining the position of the target device.        Enhanced cell ID (E-CID): It uses one or more radio measurements for determining the target device's position. The E-CID method uses at least the cell ID of a serving and/or a neighboring cell and at least one additional radio measurements which can be performed by the target device or by a radio node. For example E-CID method typically uses any combination of cell ID and radio measurements such as UE Receiver (Rx)-Transmitter (Tx) time difference, Base Station (BS) Rx-Tx time difference, Timing Advanced (TA) measured by the BS, LTE Reference Signal Received Power (RSRP) and/or Reference Signal Received Quality (RSRQ), HSPA Common Pilot Channel (CPICH) measurements, e.g. CPICH Received Signal Code Power (RSCP) and/or CPICH Ec/No), Angle of Arrival (AoA) measured by the BS on UE transmitted signals etc for determining the position of the target device. Ec/No is defined as received energy per chip (Ec) of the pilot channel divided by the total noise power density (No). The TA measurement is done using either UE Rx-Tx time difference or BS Rx-Tx time difference or both. Typically the location server uses several methods to determine the position of the target device. The E-CID positioning can also be target device assisted method or target device assisted method.        Hybrid methods: It relies on positioning measurements related to more than one positioning methods for determining the position of the target device. For example the hybrid method may use A-GNSS measurements and OTDOA RSTD measurements for determining the position of the target device. The hybrid method improves the overall accuracy of the position compared to that obtained based on an individual method.        
Positioning Architecture in LTE
The three key network elements in an LTE positioning architecture are the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e. the entities being positioned. LCS Clients may also reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the terminal or a network node or external client.
Position calculation can be conducted, for example, by a positioning server, e.g. E-SMLC or SUPL Location Platform (SLP) in LTE, where SUPL stands for Secure User Plane Location, or UE. The former approach corresponds to the UE-assisted positioning mode, whilst the latter corresponds to the UE-based positioning mode.
Two positioning protocols operating via the radio network exist in LTE, LPP and LPPa. The LPP is a point-to-point protocol between a LCS Server and a LCS target device, used in order to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. LPPa is a protocol between eNodeB and LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. SUPL protocol is used as a transport for LPP in the user plane. LPP has also a possibility to convey LPP extension messages inside LPP messages, e.g. currently Open Mobile Alliance (OMA) LPP extensions (LPPe) are being specified to allow e.g. for operator-specific assistance data or assistance data that cannot be provided with LPP or to support other position reporting formats or new positioning methods.
A high-level architecture, as it is currently standardized in LTE, is illustrated in FIG. 1 also discussed above, where the LCS target is a terminal, and the LCS Server is an E-SMLC or an SLP. In the figure, the control plane positioning protocols with E-SMLC as the terminating point are shown, and the user plane positioning protocol is shown. SLP may comprise two components, SUPL Positioning Center (SPC) and SUPL Location Center (SLC) (not shown), which may also reside in different nodes. In an example implementation, SPC has a proprietary interface with E-SMLC, and Lip interface with SLC, and the SLC part of SLP communicates with PDN-Gateway (P-GW) and External LCS Client. PDN stands for Packet Data Network. The P-GW is further connected over the Si interface to the RAN via a Serving Gateway (S-GW).
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
For UL positioning, e.g., UTDOA, also LMUs may be comprised in the positioning architecture, see FIG. 2. The LMUs may be as shown, e.g., standalone, integrated into eNodeB (eNB) or co-sited with eNB. Hence, as shown in FIG. 2, the LMU may be an integrated LMU, a standalone LMU interfacing eNB or LMU sharing eNB antenna, or a standalone LMU with own radio equipment. In LTE, UTDOA measurements, Uplink Relative Time of Arrival (UL RTOA), are performed on Sounding Reference Signals (SRS). To detect an SRS signal, LMU needs a number of SRS parameters to generate the SRS sequence which is to be correlated to receive signals. SRS parameters would have to be provided in the assistance data transmitted by positioning node to LMU; these assistance data would be provided via protocol LMUp from the E-SMLC. However, these parameters are generally not known to the positioning node, which needs then to obtain this information from eNodeB configuring the SRS to be transmitted by the UE and measured by LMU; this information would have to be provided in LPPa or similar protocol
Uplink Multi-antenna Transmission
Uplink Multiple Input Multiple Output (MIMO) and uplink transmit diversity are both example of uplink multi-antenna transmission schemes. In both cases the UE transmits uplink signals with more than one transmit antenna.
In LTE different uplink multi-antenna transmissions techniques can be applied in the uplink; for example beamforming or antenna switching. The scheme can also be open loop or closed loop. Open loop uplink multi-antenna techniques are based on the assumption that the UE does not have information about the uplink channel, hence it cannot exploit this knowledge in order to optimize the transmission weights, the transmission beamforming, in order to steer the beam in the direction of the base station. On the contrary, in case of closed loop multi-antenna techniques the UE has some information about the uplink channel which exploits in order to optimize the beamforming vector.
The uplink transmit diversity can also be regarded as a special case of uplink MIMO transmission scheme.
In any MIMO or transmit diversity scheme, a set of parameters related to MIMO or uplink transmit diversity are regularly adjusted by the UE. The objective is to ensure that the uplink transmission incorporates the desired spatial, temporal or angular diversities. This in turns improves uplink coverage, reduces interference, increases uplink bit rate and enables UE to lower its transmitted power.
The MIMO or transmit diversity parameters comprise of: relative phase, relative amplitude, relative power, relative frequency, timing, absolute or total power of signals transmitted on transmit diversity branches, etc. The adjustment of all or a sub-set of these parameters is fundamental to transmit beamforming scheme. The objective of beamforming is to direct the uplink transmission or beam towards the desired base station, which is generally the serving base station. This allows the serving base station to easily decode the received signal more easily. Furthermore, high directivity of the beam towards the desired base station reduces the interference towards the neighboring base stations. Similarly in case of switched antenna transmit diversity, transmit diversity parameter implies the selection of the most appropriate transmit antenna, e.g. in terms of radio condition, out of the available transmit diversity branches. By the virtue of using the most appropriate antenna for the uplink transmission, the UE can either reduce its power while retaining a given uplink information rate, or increase the information rate while retaining a given output power.
In open loop MIMO or transmit diversity schemes, the UE autonomously adjusts the uplink transmit diversity parameters without the use of any network transmitted control signaling or commands. These schemes are simpler, although they may not show substantial gain in all scenarios.
In case of beamforming the UE typically uses one Power Amplifier (PA) per transmit antennas. The maximum output power of each PA used for an antenna can be scaled, e.g. lowered, by the number of transmit antenna. For example a UE with 2×2 UL MIMO may limit the maximum output power of 21 dBm for each of the two PA, e.g. vs. 23 dBm with 1 transmit antenna, when operating with beamforming i.e. when transmitting with both antennas at the same time to steer the uplink transmitted beam.
Multi-Carrier or Carrier Aggregation
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MHz carriers in HSPA to enhance the peak-rate within the HSPA network. Similarly in LTE for example multiple 20 MHz carriers or even smaller carriers, e.g. 5 MHz, can be aggregated in the UL and/or on DL. Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier (CC) or sometimes is also referred to a cell. In simple words the CC means an individual carrier in a multi-carrier system. The term carrier aggregation (CA) is also called, (e.g. interchangeably called, “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the Primary Component Carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called Secondary Component Carrier (SCC) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signaling. The primary CC, aka PCC or PCell, exists in both uplink and downlink directions in CA. In case there is single UL CC the PCell is obviously on that CC. The network may assign different primary carriers to different UEs operating in the same sector or cell.
Therefore the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCC respectively. The serving cell is interchangeably called as Primary cell (PCell) or Primary Serving Cell (PSC). Similarly the secondary serving cell is interchangeably called as Secondary cell (SCell) or Secondary Serving Cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in DL and UL for the reception and transmission of data by the UE. The remaining non-serving cells on the PCC and SCC are called neighbor cells.
The CCs belonging to the CA may belong to the same frequency band, aka intra-band CA, or to different frequency band, inter-band CA, or any combination thereof, e.g. 2 CCs in band A and 1 CC in band B. The inter-band CA comprising of carriers distributed over two bands is also called as dual-band-dual-carrier-High Speed Downlink Packet Access (DB-DC-HSDPA) in HSPA or inter-band CA in LTE. Furthermore the CCs in infra-band CA may be adjacent or non-adjacent in frequency domain, aka intra-band non-contiguous CA. A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity the carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example signals on each CC may be transmitted by the eNB to the UE over two or more antennas.
The CCs in CA may or may not be co-located in the same site or base station or radio network node (e.g. relay, mobile relay etc). For instance the CCs may originate (i.e. transmitted/received) at different locations, e.g. from non-located BS or from BS and remote radio head (RRH) or remote radio units (RRU). The well-known examples of combined CA and multi-point communication are Distributed Antenna System (DAS), RRH, RRU, with Coordinated Multipoint (CoMP), multi-point transmission/reception etc.
Several existing positioning measurements are performed by the wireless devices, e.g. UE, target device etc, and/or measuring node, e.g. BS, LMU, etc, on signals transmitted by the wireless device. Some of these positioning measurements are only performed on UL transmitted signals e.g. Angle of Arrival, AoA, performed by the BS, UL RTOA performed by LMU etc, Some of these positioning measurements are performed on both DL and UL transmitted signals e.g. BS Rx-Tx time difference measurement, UE Rx-Tx time difference measurement, timing advance (TA), Round Trip Time (RTT), etc. In some cases, the performance of a positioning may be degraded when performed by a measuring node which is far from the UE location. This will lead to an inaccurate positioning measurement, which eventually degrades the positioning accuracy of UE location based on one or more of these positioning measurements.